Effects of Workstation Design on Assembly Time of...

Vol. 2, Mac 2017 Asian Journal of Technical Vocational Education And Training (AJTVET)

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Effects of Workstation Design on Assembly Time of Electrical

Socket Plugs

Isa Halim*, Sharifah Aznee**, Seri Rahayu*** and Adi Saptari**** *Faculty of Manufacturing Engineering,

Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal,

Melaka, Malaysia.

*Kementerian Pendidikan Tinggi Malaysia, Bahagian Pembangunan Sumber Manusia, Aras 15, No.2, Menara 2,

Jalan P5/6 Presint 5, 62200 Putrajaya.

Email: [email protected]

Abstract

The design of assembly workstation contributes significant effects to productivity, efficiency and comfort of

workers in electrical socket plugs industry. Hence, the purpose of this study is to compare the assembly time of

eight different workstation designs in assembling electrical socket plugs. The combination of the experiment

design and duration were deployed to measure the performance of the workstation designs. A group of 24

students participated in the experimental works. The subjects assembled the socket plugs at different setting of

operator number, arrangement of component, and working position. The results of this study found that all

workstation designs show significant difference in assembly time. This study concluded that workstation with

two operators, side component arrangement and sitting working position yielded fastest assembly time.

However, the number of operators also contributed to the assembly time significantly.

Key words : Workstation design, Assembly time, Electrical socket plugs, Operator, Posture, Component

arrangement

1. Introduction

An industrial workstation is a space for one or more operators to carry out assembly processes. Usually, the

workstation is equipped with work materials (parts and components), tools, and machine. The industrial workstation should

be designed ergonomically by taking into account the relationship between operators, materials, and tools to enable the

operators to perform the assembly process productively and comfortably. Consequently, the industry’s management needs

to prioritize the related work issues such as repetitive, boring and tiring as it affects productivity and operators’ satisfaction

(Shikdarand Das, 2003). Major emphasis on ergonomics aspect of workstation design factors such as orientation and

distance can maximize performance and quality, while minimizing physical stress (Seoungyeon and Robert, 1997). Many

studies proved that an ergonomic workstation design contributes a lot of benefits such as improvements in quality,

productivity and comfort level, and elimination of rejection cost (Yeow, 2003;Ro-Ting and Chang-Chuan, 2007). On the

other hand, failure to match the operators’ abilities with the task requirements in designing the workstation resulting in lost

operator productivity and occupational injuries (Biman and Sengupta, 1996). Previous study has shown when an operator

performs jobs in an ergonomic workstation; productivity can increase (Peter et al., 2006). In designing an ergonomic

industrial workstation, factors of human (e.g. psychophysical experience, posture and muscle activity), machine and

environment (e.g. noise level, vibration exposure, and thermal comfort) should be critically considered. There is a good

agreement found between thermal comfort and productivity performance of operators at the workstation (Li et al., 2010;

2011). Workstation design factors such as mass of tools, working height, and operator’s movement distance have been

identified as vital variables that can maximize productivity and performance, and reduce costs (Resnick and Zanotti, 1997).


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A study on assembly line systems found that percentage of improvement in productivity tends to be greater during high

demand periods when operators work at U-shape line compared to a straight-line assembly system (Gerald e al., 2004).

In manufacturing industries, jobs can be categorized into precision and heavy job. For precision job, the

recommended working height is at above elbow height. On the other hand for heavier work, making use of the weight of the

upper part of the body in order to exert a hand force that is directed more or less downwards, a working height below elbow

height is recommended (Nico and Jan, 2002). Previous study proposed that the working height for different kinds of jobs

are: precision job for men should be set at 100-110 cm, light work around 90-95 cm and heavier work around 75-90 cm

(Grandjean, 1998). In the workstation, the job can be performed in standing and/or sitting. Standing workstation is applied

when the job requires worker move frequently, handle heavy or large objects, and exert large forces. However, performing

job in prolonged time periods can lead to body discomfort and muscle fatigue (Isa et al., 2012). Meanwhile, a seated

workstation is chosen usually for long-term duration job. A seated workstation allows better controlled on arm movements,

hence provides a stronger sense and balance.

Various research works have been performed to establish a workstation that can accommodate operators’

requirements and industry productivity. Concurrently with the fast growing of technology and research in industrial

engineering, there are several methods and tools have been developed to measure and analyze the performance of

workstation design. The methods can be categorized into subjective and direct technical measurement methods, and they

can be used either at actual workstation in industry or in laboratory setting. Subjective method is used to obtain

psychological feedbacks from the workers regarding the workstation design. Normally the subjective method is applied

through personal interview and questionnaire survey. Dissimilar to subjective method, direct technical measurement method

measures and analyses physiological and biomechanical responses of workers due to workstation design. Basically, this

method produces specific quantities such as frequency, distance, and temperature. The advantages of utilizing direct

technical measurement method are reliable data that can represent the actual condition of subjects (workers) during the

experimental works. Examples of tools include surface electromyography and oxygen consumption analyzer (Isa et al.,

2012;Sandra et al, 2012). Videotaping assessment is commonly deployed for collecting data and information such as plant

layout, process flow, movement and activities of operators in industrial workplaces (Javier et al., 2007). Besides, motion

and time study have been adopted to improve productivity of motor vehicle inspection (Khalid, 2011). In experimental

work, design of experiment (DoE) is commonly applied. For instance, DoE has been applied to study eight different

workstations for hydraulic hoses assembly process (Cimino et al., 2009).

A study shows that the use of hand tool (jig) and table height have contributed significantly to the cycle time of

electrical socket plugs assembly (Saptari et al., 2015); however, another key factor which is equally significant in

influencing the assembly time of electrical socket plugs is the arrangement of socket plug components. The arrangement of

components can either be a straight flow, side by side, and U-shape. These components arrangements are meaningful to be

studied. Hence, this study is performed to measure and compare the assembly time of different workstation designs consist

of number of operators, arrangement of components, and working position in electrical socket plugs assembly. These

findings will certainly have positive impact on enterprises which carry out the assembly process of electrical socket plugs.

They can maximize productivity by organizing the number of operators, components arrangement and working position in

workstation.

2. Experimental

The product selected for the experimental works is electrical socket plugs. Each socket plug has eight components:

female cover, neutral pin, earth pin, live pin, fuse holder, fuse, male cover and one screw. A manual screw driver is

provided to assemble the socket plugs. The experiment laboratory is equipped with excellent air conditioner noise control

and also good indoor air quality. In addition, eight assembly tables and stools are provided to run the experiment. The

assembly processes of the socket plugs are described as follow:

One component of the socket plugs is located into one assembly box. There were eight socket plug components in

the separated eight assembly boxes: Box 1-female covers of the socket plug; Box 2-the earth pins; Box 3-the neutral pins;


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Box 4-the live pins; Box 5-the fuse holders; Box 6-the fuses; Box 7-the male covers of the socket plug; Box 8-the screws.

The assembly boxes were located on the top of the table and arranged from left to right according to the following assembly

processes:

i. Place the female cover of the socket plug on the table.

ii. Insert the earth pin into the top center rectangular hole of the female cover.

iii. Insert the neutral pin into the bottom rectangular hole on the left female cover.

iv. Insert the live pin into the bottom rectangular hole on the right female cover.

v. Insert the fuse holder in the female cover.

vi. Insert the fuse into fuse holder.

vii. Attach the male cover to female cover of the socket plug. These two covers are pressed together to ensure they firmly

fixed.

viii. Insert the screw into the center round hole of the female cover.

ix. The final step is tightening the screw in clockwise direction using a screw driver. The assembly time was recorded

from the first process until the last process. Figure 1 depicts the assembly processes of the socket plugs.

Fig1 Arrangement and assembly process flow of the electrical socket plugs (a), actual workstation design (b)

A group of 24 undergraduates from third year engineering program had participated as subjects in the experimental

work. Before the experiment was conducted, each subject was given sufficient time to get enough practice to familiarize

with the experimental procedures. The subject was also informed that he / she has to perform the experiment at their own

pace.

Eight workstation designs were tested. The settings of workstations are described in Table 1. 2.1 Design of experiment

Three independent variables (factors) were tested in the experiment. The factors are number of operators

(OPERATOR), arrangement of components (ARRANGEMENT), and working positions (POSITION). Each factor has two

levels, OPERATOR: One Operator and Two Operators, ARRANGEMENT: U-shape and Side by side, and POSITION: Sit

and Stand. In total, the experiment setting has 2 x 2 x 2 combinations which are equal to eight combinations of factors and

levels. The dependent variable is assembly time, measured in sec. Table 1 shows the experiment design.


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Table 1 Design of experiment to measure the assembly time at different workstation design

Based on the above experiment design, this study investigates the difference in assembly time for each workstation

designs, and factors that contributed significantly to assembly time.

The experiment setting is randomly selected from the design of experiment to avoid unfavourable opinion. In

addition, 20 replications of socket assembly in each workstation design were performed by the group of operators to get a

stable and representative performance of assembly time. Therefore, 20 replications in eight workstation designs yielded 160

tests. Statistical analysis tools include descriptive statistics, F-test, t-test, and main effects plot were used to analyze the data

of assembly time. In addition, P-value < 0.05 is defined as significant difference.

3. Results and Discussion

Workstation 1 obtained the highest mean of assembly time (25.8 sec). The mean of assembly time for Workstation 2

is slightly lower than Workstation 1 (24.6 sec). In contrast, the Workstation 7 recorded the lowest mean of assembly time

(21.4 sec). Table 2 tabulates the minimum, mean, maximum, variance and standard deviation of assembly time for all

workstations. The Workstation 1 was operated by one operator with the components of the electrical socket plugs were

arranged in U-shape, and the working position was sitting. Meanwhile the Workstation 7 was handled by two operators, the

components of the electrical were arranged in side by side of straight-line layout, and the working position was sitting. The

different features between the both workstations are number of operator (one operator versus two operators), and the layouts

of the components arrangement (U-shape versus side by side).

This section justifies the effects of number of operators and layout of component arrangement on

assembly time. Through this experimental works, this study found that there are advantages of having more

operators. In this case, the two operators can share the components to be assembled. This practice enables them

to be more focused as they worked with minimum number of components. Consequently the cycle time can be

reduced. However the increasing number of operators does not necessarily increase the production cost per

finished product. This is due to cost reduction as a result of doing focused work. In other words, each operator

assembles less components to complete a cycle time will result in operator becoming more focused when doing

their work compared to operators doing more assembly work to finish a cycle time. This ultimately optimizes

efficiency and productivity and consequently reduces the production cost per unit produced.

This study identified that the U-shape layout is not always contributing good assembly time even though

this layout has been proven more productive than a straight-line layout (Gerald et al. 2004). In this experimental

work, the first and second operator of Workstation 7 distributed eight socket plug components equally. Four

components were arranged into four separate boxes for each operator. By doing so, the cycle time for each

operator becomes faster. Then the four boxes were arranged in straight-line layout at their side. The operator

who sits on the left side placed the boxes at his left, and the operator who sits on the right side placed the boxes

at his right. Once the first operator had assembled the first four components, the second operator proceeded to

OPERATOR One Operator Two Operators

ARRANGEMENT U-shape Side U-shape Side

POSITION Sit

Sta

nd

Sit

Sta

nd

Sit

Sta

nd

Sit

Sta

nd

Workstation

Wst

1

Wst

2

Wst

3

Wst

4

Wst

5

Wst

6

Wst

7

Wst

8


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assemble another four components. This study observed that when the components are positioned at side by side,

the operator can reach and grab the components easily. Additionally the minimum length of the boxes

contributed to less assembly time.

Table 2 Minimum, mean, maximum, and standard deviation (SD) of assembly time for each workstation

Workstation Min Mean Max Variance SD

W 1 14.47 25.80 55 40.9 6.4

W 2 15 24.6 64 35 5.9

W 3 14 23.6 53.8 28.6 5.3

W 4 14.3 23.5 48.1 25.9 5

W 5 13 22.5 45 26.9 5.2

W 6 14.3 22.2 51 15.3 3.9

W 7 15 21.4 43.3 10.4 3.2

W 8 12 23.1 47.3 33 5.8

Comparative statistics associated with Analysis of Variance (ANOVA) and t-test were performed to find significant

difference among the workstation designs. As tabulated in Table 3, the one-way ANOVA found that there is a significant

difference among the eight workstation designs as verified by P-value = 2.13E-47 < 0.05.

Table 3 Results of single factor ANOVA

This study observed any significance difference of assembly time through one-to-one comparison of the

workstation designs. Comparative statistics associated with t-test was applied and the results are tabulated in

Table 4. The results found that there is a significant difference in assembly time; however, assembly time of

Workstation 3 and Workstation 4, Workstation 5 and Workstation 6, and, Workstation 3 and Workstation 8 did

not show any significant difference.

Source of

Variation

SS df MS F P-value F crit

Between

Groups

6583.3

02

7

940.4

718

34.8

1737

2.1

3E

-47

2.0

11969

Within

Groups

103508.3

3832

27.0

1157

- - -


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Table 4 Comparison of assembly time among the workstation designs

Note: F-value at the top, P-value in the parentheses

Three-way Analysis of Variance (ANOVA) was applied to investigate which factors contribute more

significant than the others. Table 5 tabulates the ANOVA results whereby factors of ARRANGEMENT (U-

shape and side) and OPERATOR (one operator and two operators) are significantly contribute to assembly time.

Between the two, OPERATOR (F = 155.52) is the most significant influencing the assembly time. On the other

hand, POSITION (either sitting or standing) does not significantly influence the assembly time. In addition,

combination of ARRANGEMENT and POSITION (F = 21.96) shows a significant effect to the assembly time,

whereas combination of ARRANGEMENT, POSITION and OPERATOR (F = 1.61) did not significantly

contribute to assembly time.

Work

station

Wst

2

Wst

3

Wst

4

Wst

5

Wst

6

Wst

7

Wst

8

Wst

1

1.1

67

[0.0

03]

1.4

3

[1.0

E-

8]

1.5

8

[2.7

E-

9]

1.5

2

[2.9

E-

18

] 2

.67

[7.2

E-

24

] 3

.95

[1.9

E-

37

] 1

.24

[2.4

E-

11

]

Wst

2

-

1.2

3

[0.0

06]

1.3

5

[0.0

03]

1.3

0

[2.8

E-

9]

2.2

8

[5.7

E-

13

] 3

.38

[2.5

E-

24

] 1

.06

[0.0

00

1]

Wst

3

- -

1.1

[0.8

9]

1.0

6

[0.0

00

7

] 1

.86

[7.1

8E

-

6]

2.7

6

[1.3

E-

14

] 0

.86

[0.2

0]

Wst

4

- - - 0.9

6

[0.0

00

9]

1.6

9

[7.3

7E

-6]

2.4

9

[5E

-

15]

0.7

8

[0.2

4]

Wst

5

- - - -

1.7

5

[0.4

6]

2.5

9

[9.8

8E

-

5]

0.8

1

[0.0

51]

Wst

6

- - - - - 1.4

8

[0.0

0017

] 0.4

6

[0.0

04]

Wst

7

- - - - - - 0.3

1

[5.0

3E

-

9]


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Table 5 ANOVA - Time versus ARRANGEMENT, POSITION, OPERATOR

Source DF SS MS F P

ARRANGEMENT 1

70

6.1

3

70

6.1

3

26

.14

0.0

00

POSITION 1

6.4

2

6.4

2

0.2

4

0.6

26

OPERATOR 1

42

00

.97

42

00

.97

15

5.5

2

0.0

00

ARRANGEMENT

*POSITION 1

59

3.2

3

59

3.2

3

21

.96

0.0

00

ARRANGEMENT

*OPERATOR

1

564.0

8

564.0

8

20.8

8

0.0

00

POSITION*OPER

ATOR

1

468.9

9

468.9

9

17.3

6

0.0

00

ARRANGEMENT

*POSITION*OPE

RATOR

1

43.4

9

43.4

9

1.6

1

0.2

05

A mathematical model from regression analysis was established. As shown in Table 6, the model

expresses the Assembly time = 27.8 – 2.09 OPERATOR – 0.858 ARRANGEMENT where the number of

operator has a significant, negative effect to assembly time. This model proved that the number of operator

contributes vital effect to assembly time as this factor has largest coefficient. This model validates the finding of

ANOVA.

Table 6 Regression Analysis - Time versus OPERATOR, ARRANGEMENT, POSITION

Predictor Coef SE Coef T P

Constant 27.7728 0.3683 75.41 0.000

OPERATOR -2.0919 0.1690 -12.38 0.000

ARRANGEMENT -0.8576 0.1690 -5.08 0.000

S = 5.23576 R-Sq = 4.5% R-Sq(adj) = 4.4%

Note: POSITION is highly correlated with other X variables and it has been removed from the equation.

The main effects plot for assembly time was developed to map the independent variables (OPERATOR,

ARRANGEMENT and POSITION) in different levels (OPERATOR: one operator and two operators;

ARRANGEMENT: U-shape and side; POSITION: sitting or standing) with respect to mean of assembly time

(dependent variable). Figure 2 shows the individual effect of OPERATOR, ARRANGEMENT and POSITION


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corresponding to assembly time, where the bigger slope indicates the bigger effect in the assembly process. The

main effects plot indicates that:

• The assembly time of electrical socket plugs decrease when deploying more operators. In this case, two

operators resulted in lower assembly time compared to one operator.

• Proper arrangement of the components can increase the assembly time. The result found that assembly

time is lesser when the components are arranged in side by side, as opposed to U-shape.

• Working position whether standing or sitting has an effect to assembly time. This experiment revealed that

standing position is more productive than sitting.

Based on the main effects plot, the slope of OPERATOR is the biggest. It indicates the number of operator

is the main effect for assembly time. On the other hand, the slope of ARRANGEMENT and POSITION has

equal size and smaller than OPERATOR slope. It indicates ARRANGEMENT and POSITION contribute minor

effect on assembly time.

Fig 2. Main effects plot for assembly time of electrical socket plugs

4. Conclusion

Based on the experimental work, this study concluded that there is a significant difference in assembly time among

the eight workstation designs of electrical socket plug. The workstation which is designed with two operators, side

component arrangement and sitting working position has yielded fastest assembly time. In contrast, workstation which is

designed with one operator, U-shape component arrangement and sitting working position resulted in slowest assembly

time. Additionally, this study proved that the number of operator has contributed significantly to assembly time.

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